Method of prepamng molybdenum



United States Patent 3,125,413 METHOD OF PREPARING MOLYBDENUM DISULFIDE Edgar K. Leavenworth, Coldwater, Mich, and Louis S.

Deitz, Jr., Westfield, N.J., assignors to American Metal gliniax, Inc., New York, N.Y., a corporation of New or No Drawing. Filed Aug. 24, 1960, Ser. No. 51,524

14 Claims. (Cl. 23-134) This invention relates to a method of preparing molybdenum disulfide powder, and more particularly to a controlled retorting process for preparing molybdenum disulfide powders containing a controlled quantity of a residuary oily substance therein to prevent oxidation of the molybdenum disulfide particles. This application is a continuation-in-part of prior copending application Serial No. 770,308, filed October 29, 1958, and entitled Methods of Treating Molybdenum Disulflde, now abandoned.

In the past, commercial molybdenum disulfide has been produced by grinding molybdenite ore comprised largely of granite containing molybdenum disulfide and thereafter separating the molybdenum disulfide from the gangue by flotation techniques involving the use of petroleum oils and other oily substances of various kinds. The process is one in which repeated grinding and flotation operations are carried out until the amount of the gangue is reduced to the desired level. This gangue which is usually identitled as the portion insoluble in nitric acid and which is largely silica, is referred to hereinafter as silica. It is all soluble in hydrofluoric acid. In the usual commercial flotation extraction processes, the silica content of the molybdenum disulfide powder is reduced to a level below 12%. However, when high purity is desired as in the case of a molybdenum disulfide lubricant it is possible to reduce the silica content of the powder to an amount ranging from about .3% to about .5 by repeated grinding and flotation extraction until the ore has an average particle size ranging from about to about 250 microns. It is usually not economical to reduce the silica content any further by the grinding and flotation process.

The product derived from the flotation process comprises a wet oily mass of molybdenum disulfide powder usually containing up to about 16% water and up to about 7% flotation oils. In the past it has been the practice to evaporate and thus remove the oil and water by heating in a retort at 1200 F. until the powder contained no more than .03% oil and .03% water. It should be understood that the term evaporate as here used as to the oil usually includes some cracking of the oil to produce products which are gaseous at the temperature employed. The resultant. dry and substantially oil-free powder ob-' tained from the retorting process is susceptible to oxidation on aging in the presence of air forming oxides of molybdenum and sulfur, which impart acidity to the powder which is particularly objectionable when it is to be used for lubrication purposes.

It has been recognized that the coating of all the particles in the powder with an oily substance in concentrations as low as .05% will provide protection against oxidation for substantially long periods of time. It has been proposed in United States Patent No. 2,686,156 that a small quantity of oil be added to a molybdenum disulfide powder to stabilize it against oxidation. How ever, the process disclosed in this patent constitutes an expensive operation since in order to coat the powder uniformly with such a small quantity of oil it is necessary to first dissolve the oil in a solvent before mixing it with the powder and thereafter removing the solvent by evaporation. Accordingly, a need has heretofore existed for a practical and inexpensive method of providing a coating of an oily substance on molybdenum disulfide powders which overcomes the problem presented by the fact that small quantities of oil cannot be applied uniformly to all of the small particles by ordinary mixing techniques.

In addition, for some lubrication purposes it is preferred to subject the molybdenum disulfide powder to a further pulverizing process which reduces the average particle size to a magnitude, for example, below about 5 microns. Heretofore, the pulverization of substantially oil-free molybdenum disulfide powders caused an objectionable degree of oxidation and acidity to occur and the resultant finely comminuted powder was extremely susceptible to further oxidation when exposed to the atmosphere.

It is a general object of the present invention to provide an improved method of treating molybdenum disulfide powder which is more economical than those previously used, and which produces a powdered product having a controlled residuary oil content which inhibits oxidation of the particles.

The foregoing and other objects of this invention are achieved by subjecting the oil flotation extracted molybdenum disulfide powder containing up to about 16% water and up to about 7% flotation Oils to a controlled retorting operation whereby the water is reduced to a value below about .5% and the residuary oil content of the powder is simply and accurately reduced to a level rang ing from about 0.05% to about 6%. The retorted molybdenum disulfide powder product having a coating of an oily substance in a controlled amount can thereafter be subjected to impact pulverization in a fluid energy mill for further comminution of the powder.

In accordance with the present invention, the molybdenite ore is subject to a conventional flotation extraction process, whichis carried out until the silica content of the powdered ore is less than 8% and preferably from about .3% to .5%. The end product of this process, after the bulk of the water has been filtered out, is a wet, oily mass of powder containing about 16% water and up to about 7% flotation oils. While a wide variety of oils and oily substances are used in the conventional flotation extraction process and may be employed for purposes of the present invention, it is preferred that they include some fairly high boiling point oils (i.e., oils having an end boiling point above 250 C.), since otherwise it is difficult to control the end point of the retorting operation or retain adequate residual oil in the powder. Any oily substances of vegetable or petroleum origin which have boiling points in the range of 250 C. to 650 C. and which will wet the powder may be employed. For example, a mixture of two parts kerosene to one part pine tar oil gives satisfactory results, while pine oil alone does not leave sufficient oily residue. Mixtures of the lower boiling point materials such as pine oil with the higher boiling point materials such as pine tar oil, kerosene, or other higher boiling petroleum flotation oils will give satisfactory results.

If a final product having less silica than that remaining in the product of the flotation process is desired, the wet oily powder can be subjected to an intervening leaching step whereby the silica. content is reduced to a range from about 0.01% to about 0.05% without reducing the oil continuous basis. Equipment which may be employed in the continuous retorting process consists of a cylindrical rotating retort and a refractory-lined cylindrical furnace which completely surrounds the retort for heating the material to be heated. The material is fed continuously into the retort at one end and discharged continuously at the other by means of screw conveyors. Vibrating or rotary feeders may also be employed in the charging process which is performed in such a manner that there is no interchange between the heating atmosphere and the outside air. The size of the retort is not important but such retorts customarily range in size between 6 inches and 48 inches in diameter, and from 4 to 30 feet in length. During the continuous retorting process an inert gaseous atmosphere is usually introduced into the retort area, passed over the material being processed and then exhausted. This gaseous atmosphere serves the function of preventing oxidation of the material being treated, and also serves to carry away the vapors which have been driven off the material during the heating process. Examples of suitable inert gases for this purpose are nitrogen, the product obtained by burning oil or gas in a Kemp generator which is largely a mixture of CO and nitrogen, or a mixture of the foregoing with recycled spent gases from the retorting operation.

It will be appreciated that for a given furnace, quantity of initial oil, and time and temperature of treatment, an increase in the purge rate decreases the residual oil and vice versa. Similarly, an increase in either time or temperature of treatment will decrease the residual oil, other factors remaining the same. Furnace design and construction will also have some effect.

The retorting treatment involves reducing the residual oil by evaporation (including cracking) to a value within any desired range depending on the end use to which the molybdenum disulfide powder is to be put, but not less than 0.05 since inadequate oxidation protection is obtained at oil contents below 0.05%. This treatment also reduces the water content to a negligible value, usually less than about 0.5%. When the molybdenum disulfide powders are to be utilized in the preparation of dry" lubricants, it is preferred that the powder be of a seemingly dry, free-flowing nature and necessarily contains only a small quantity of residuary oil to avoid impairment of fiowability. Seemingly dry, free-flowing powders can ordinarily contain residuary oil contents ranging from 0.05% to about 0.5%, and preferably from 0.05% to about 0.2%. Oil contents in excess of 0.5% impair the free-flowing characteristic of the powder but may be satisfactorily utilized in compounding with lubricating oils and greases. Accordingly, the retorting process can be controlled to produce substantially water-free molybdenum disulfide powders containing from about 0.05% to about 6 of the residuary oil depending on the end use of the product.

For any given furnace and starting material the time and temperature of treatment and the purge rate to leave the residual oil content at a level within the range desired may be readily determined by trial in view of the following results obtained with powder containing 5% oil and water in a continuous rotary furnace which was 6 inches in diameter and 7 feet long.

The resultant molybdenum disulfide powder derived from the controlled retorting process and containing the desired residuary flotation oil content can be used in that form or may be subject to further treatment. The powder obtained from the retorting process usually is of an average particle size ranging from about 10 to about 250 microns as determined by the degree of pulverization of the molybdenite ore during the flotation extraction process. Although molybdenum disulfide particle sizes within the aforementioned range are satisfactory for many purposes, molybdenum disulfide powders for use in lubricants are preferably of a smaller particle size. Accordingly, when extremely fine molybdenum disulfide powders are desired, the product from the controlled retorting process can be subjected to impact pulverization in a fluid energy mill wherein the particles are further reduced in size by collision and abrasion with each other at high velocity while moving in a stream of gas. Impact pulverization mills suitable for this purpose are made by Fluid Energy Processing and Equipment Company of Philadelphia, Pennsylvania, and by Sturtevant Mill Company of Boston, Massachusetts. Impact pulverization, or micronization as it is usually referred to, can be controlled to achieve any particle size and the particles when attaining that size are removed from the larger particles through centrifugal action. The molybdenum disulfide powder derived from the controlled retorting operation is generally of an average particle size ranging from about 10 to about 250 microns and is reduced by the impact pulverization operation to an average particle size such as, for example, of about 5 microns or less. When the resultant powder derived from the controlled retorting operation is to be subjected to further comminution in the impact pulverization mill it is preferred that the residual oil content thereof is above the minimum limit of 0.05 such as, for example, at least about .15 to about 3%.

One of the highly advantageous features of the present invention is that the molybdenum disulfide powder derived from the controlled retorting operation and containing from about .15% to about 6% residual oil, will give excellent protection against oxidation during the impact pulverization and will also stabilize the resultant pulverized powder product against oxidation for substantial periods of time thereafter. This factor is quite significant when it is considered that the direct coating of the molybdenum disulfide powder with such small quantities of oil by ordinary mixing techniques is virtually impossible and further, that the surface area of the powder is increased many times during the pulverization. The character of the protection achieved by the residuary oil coating may be seen from the following table which shows the quantity of M00 in powder which has been impact pulverized to a particle size of approximately one-half micron diameter and stored in open air for varying periods.

Percent M00 Sample No. Percent 3 Oil At Start 1 Month 2 Months none .351 512 The difference in M00 content between samples 1 and 5 at the start of aging gives some indication of the amount of oxidation which occurs during micronizing when the powder contains no oil. For use as a dry lubricant, sample 3 would be satisfactory under many circumstances. Samples 1 and 2 are of course superior and samples 4 and 5 unsatisfactory.

In accordance with the method herein disclosed, substantially dry molybdenum disulfide powders containing a controlled coating of a residuary oily substance ranging from at least 0.05% to about 6% can be simply and inexpensively produced and which are characterized by their exceptionally high stability against oxidation in the presence of air for long periods of time. The molybdenum disulfide powders derived from the controlled retorting operation and containing a residuary oil content in the aforementioned range can be subjected to impact pulverization in a fluid energy mill thereby producing an exceedingly fine molybdenum disulfide powder which is stable, and resistent to oxidation on exposure to the atmosphere. The method disclosed is particularly useful for preparing exceedingly fine, seemingly dry, free-flowing molybdenum disulfide powders having residuary oil contents ranging from .05% to about .5% which are ideal for use as solid lubricants.

It will be understood that all percentages heretofore given in the specification and set forth in the subjoined claims, unless otherwise specified, are expressed in terms of percentages by weight.

What is claimed is:

l. The method of producing a molybdenum disulfide powder having a controlled quantity of la residuary oily substance comprising the steps of providing a powdered mass of molybdenum disulfide containing up to about 16% water and up to about 7% of an oily substance, heating said powdered mass in a retorting operation and controlling the temperature and retort time to reduce the water content to a level below about 0.5 and the oily substance to a level ranging from 0.05 to about 6% by evaporation.

2. The method of producing a molybdenum disulfide powder having a controlledquantity of a residuary oily substance comprising the steps of providing. a powdered mass of molybdenum disulfide containing up to about 16 water and up to about 7% of an oily substance, said oily substance comprising an oilselected from the group consisting of petroleum oils,. vegetable oils, and mixtures thereof which wet the particles. and including at least a portion that has an end boiling point above 250 C., heating said powdered mass in a retorting operation and controlling the temperature and retort time to reduce the water content to a level below about 0.5% and said oily substance to a level ranging from 0 .05% to about 6% by evaporation.

3. The method of producing a molybdenum disulfide powder containing a controlled quantity of a residuary oil comprising the steps of extracting by oil flotation from powdered molybdenite ore a molybdenum disulfide powder containing up to about 16% water and up to about 7% flotation oil, and thereafter heating said powder in a retorting operation and controlling the temperature and retort time to reduce the water content to a level of less than 0.5 and the oil content of the powder to a level ranging from 0.05 to about 6% by evaporation.

4. The method of producing a molybdenum disulfide powder containing a controlled quantity of a residual flotation oil comprising the steps of extracting by oil flotation from powdered molybdenite ore a molybdenum disulfide powder containing up to about 16% water and up to about 7% flotation oil, at least a portion of said flotation oil having an end boiling point above 250 C., and thereafter heating said molybdenum disulfide powder in a retorting operation and controlling the temperature and rotart time to reduce the water content to a level below about 0.5% and said flotation oil to a level ranging from at least 0.05% to about 6% by evaporation.

5. The method of producing a seemingly dry free-flowing molybdenum disulfide powder containing a controlled quantity of a residuary oil comprising the steps of extracting by oil flotation from powdered molybdenite ore a wet oily mass of molybdenum disulfide powder containing up to about 15% water and up to about 6% flotation oil, at least a portion of said flotation oil having an end boiling point above 250 C., and thereafter heating said powdered mass in a retorting operation and controlling the temperature and retort time to reduce the water content to alevel below about 0.5% and the flotation oil content to a value ranging between .05% and about .5% by evaporation.

6. The method of producing a seemingly dry free flowing molybdenum disulfide powder containing a controlled quantity of. a residuary oil comprising the steps of extracting by oil flotation from powdered molybdenite ore a wet oily mass of molybdenum disulfide powder containing up to about 16% water and up to about 7% flotation oil, at least a portion of said flotation oil having an end boiling point above 250 C., and thereafter heating said powdered mass in retorting operation and controlling the temperature and retort time to reduce the water content to a level below about 0.5 and the flotation oil content to a value ranging between .05 and about .5% by evaporation.

7. The method of producing a molybdenum disulfide powder having a controlled quantity of a residuary oil comprising the steps of extracting by oil flotation from powdered molybdenite ore a wet oily molybdenum disulfide powdered mass having an average particle size ranging from about 10 to about 250 microns and containing up to about 16% water and up to about 7% flotation oil, at least a portion of said flotation oil having an end boiling point above 250 C., and thereafter heating said powdered mass in a retorting operation and controlling the temperature between about 800 F. to about 1450 F. under an inert atmosphere and controlling the retort time to reduce the water content of said powdered mass to a level below about 0.5 and to reduce said flotation oil content of said powdered mass to a level ranging from 0.05% to about 6% by evaporation.

8. The method of producing a molybdenum disulfide powder having a controlled quantity of a residuary oil comprising the steps of extracting by oil flotation from finely powdered molybdenite ore a wet oily molybdenum disulfide powdered mass having an average particle size ranging from about 10 to about 250 microns and containing up to about 16% water and up to about 7% flotation oil, at least a portion of said flotation oil having an end boiling point above 250 C., and thereafter heating said powdered mass in a retorting operation and controlling the temperature between about 800 F. to about 1450 F. under an inert atmosphere and controlling the retort time to reduce the water content of said powdered mass to a level below about 0.5% and to reduce said flotation oil content of said powdered mass to a level ranging from 0.05% to about .5% by evaporation.

9. The method of producing a molybdenum disulfide powder suitable for use as a lubricant and having a controlled quantity of a residuary oily substance comprising the steps of providing a powdered mass of molybdenum disulfide containing up to about 16% water and up to about 7% of an oily substance, heating said powdered mass in a retorting operation and controlling the temperature and retort time to reduce the water content to a level below about 0.5% and the oily substance to a level ranging from 0.05 to about 6% by evaporation, and thereafter pulverizing said powdered mass in a fluid energy mill to further reduce the particle size thereof.

10. The method of producing a molybdenum disulfide powder containing a controlled quantity of a residual flotation oil comprising the steps of extracting by oil flotation from powdered molybdenite ore a molybdenum disulfide powder containing up to about 16% water and up to about 7% flotation oil, at least a portion of said flotation oil having an end boiling point above 250 C., heating said molybdenum disulfide powder in a retorting operation and controlling the temperature and retort time to reduce the water content to a level below about 0.5% and said flotation oil to a level ranging from at least 0.05 to about 6% by evaporation, and thereafter pulverizing said molybdenum disulfide powder in a fluid energy mill to further reduce the particle size thereof.

11. The method of producing a molybdenum disulfide powder having a controlled quantity of a residuary oil comprising the steps of extracting by oil flotation from powdered molybdenite ore a wet oily molybdenum disulfide powdered mass having an average particle size ranging from about to about 250 microns and containing up to about 16% water and up to about 7% flotation oil, at least a portion of said flotation oil having an end boiling point above 250 C., heating said powdered mass in a retorting operation and controlling the temperature between about 800 F. to about 1450 F. under an inert atmosphere and controlling the retort time to reduce the water content of said powdered mass to a level below about 0.5% and to reduce said flotation oil content of said powdered mass to a level ranging from 0.05% to about 6% by evaporation, and thereafter pulverizing said powdered mass in a fluid energy mill thereby reducing the average particle size to a value less than about five microns.

12. The method of producing a seemingly dry freeflowing molybdenum disulfide powder containing a controlled quantity of residuary oil comprising the steps of extracting by oil flotation from powdered molybdenite ore a wet oily mass of molybdenum disulfide powder containing up to about water and up to about 6% flotation oil, at least a portion of said flotation oil having an end boiling point above 250 C., heating said powdered mass in a retorting operation and controlling the temperature and retort time to reduce the water content to a level below about 0.5% and the flotation oil content to a value ranging between .05 and about .5 by evaporation, and thereafter pulverizing said powdered mass in a fluid energy mill thereby further reducing the particle size thereof.

13. The method of producing a seemingly dry freeflowing molybdenum disulfide powder containing a controlled quantity of a residuary oil comprising the steps of extracting by oil flotation from powdered molybdenite ore a wet oily mass of molybdenum disulfide powder containing up to about 16% water and up to about 7% flotation oil, at least a portion of said flotation oil having an end boiling point above 250 C., heating said powdered mass in a retorting operation and controlling the temperature and retort time to reduce the water content to a level below about 0.5% and the flotation oil content to a value ranging between .05% and about .5% by evaporation, and thereafter pulverizing said powdered mass in a fluid energy mill thereby further reducing the particle size thereof.

14. The method of producing a molybdenum disulfide powder suitable for use as a lubricant having a controlled quantity of a residuary oil comprising the steps of extracting by oil flotation from finely powdered molybdenite ore a wet oily molybdenum disulfide powdered mass having an average particle size ranging from about 10 to about 250 microns and containing up to about 16% Water and up to about 7% flotation oil, at least a portion of said flotation oil having an end boiling point above 250 C., heating said powdered mass in a retorting operation and controlling the temperature between about 800 F. to about 1450 F. under an inert atmosphere and controlling the retort time to reduce the water content of said powdered mass to a level below about 0.5 and to reduce said flotation oil content of said powdered mass to a level ranging from 0.05% to about .5% by evaporation, and thereafter pulverizing said powdered mass in a fluid energy mill thereby reducing the average particle size to a value less than about five microns.

References Cited in the file of this patent UNITED STATES PATENTS 2,367,946 Kaercher Jan. 23, 1945 2,686,156 Arntzen Aug. 10, 1954 2,885,154 Eastman et al. May 5, 1959 2,916,213 Paull Dec. 8, 1959 

1. THE METHOD OF PRODUCING A MOLYBDENUM DISULFIDE POWDER HAVING A CONTROLLED QUANTITY OF A RESIDUARY OILY SUBSTANCE COMPRISING THE STEPS OF JPROVIDING A POWDERED MASS OF MOLYBDENUM DISULFIDE CONTAINING UP TO ABOUT 16% WATER AND UP TO ABOUT 7% OF AN OILY SUBSTNACE, HEATING SAID POWDERED MASS IN A RETORTING OPERATION AND CONTROLLING THE TEMPERATURE AND RETORT TIME TO REDUCE THE WATER CONTENT TO A LEVEL BELOW ABOUT 0.5% AND THE OILY SUBSTANCE TO A LEVEL RANGING FROM 0.05% TO ABOUT 6% BY EVAPORATION. 